Women with BRCA1 mutations face lifetime risks of 80 percent for breast cancer and 40 percent for ovarian cancer. Despite of great effort in elucidating its functions since its cloning in 1994, the mechanisms by which BRCA1 functions in normal cells and suppresses tumor formations in breast and ovarian tissues are largely not understood. The long-term goal of this project is to understand how BRCA1 confers its biological functions in normal cells and in response to DNA damage and furthermore, how the abolishment of these functions by BRCA1 mutations leads to cancer. Two functions of the BRCA1 protein are implicated: 1) BRCA1 is involved in DNA damage repair and 2) BRCA1 is involved in gene transcriptional regulation. The principal investigator hypothesized that the BRCA1 protein serves as a scaffold protein to organize a protein complex with the build-in components of DNA damage-sensing, signaling and repair. This complex serves an indispensable role for monitoring and maintaining the genome. The applicant's preliminary data show that the BRCA1 protein exists in large protein complexes (> 2 MDa). They propose to use a combination of biochemical and physical approaches to purify and define the BRCA1 protein complexes that carry out its biological function both under constitutive conditions and in response to DNA damage. The applicant will use new mass spectrometry techniques, which offer unprecedented sensitivity and speed to aid the purification and the identification of the protein complex. This endeavor will greatly facilitate the understanding of the BRCA1 function by providing a more detailed picture of the BRCA1 biochemical pathway and ultimately, make a large impact in the understanding of breast cancer by delineating the BRCA1 pathway. The specific aims of this proposal are: (1) defining the constitutive BRCA1 protein complexes, (2) characterizing the constitutive BRCA1 protein complexes, and (3) defining the BRCA1 protein complexes in response to DNA damage.